Ab Initio Density Functional Theory Charge Distributions of Vinyltrimethylphosphonium, Vinylphosphorinium, and Vinyldiphosphonium Tetrafluoroborate Salts with Mechanistic Studies of Phospha-Michael Additions to Activated Acetylenes

Abstract

While vinylammonium and vinylpyridinium tetrafluoroborate salts have previously been well investigated by the Buszek research group in their applications to Diels-Alder and other reactions, very little is known about their phosphorus-containing derivatives, which in limited ab initio and synthetic efforts have yielded unique properties and deviations. Efforts to explore the extension to vinylphosphonium and other phosphorus containing tetrafluoroborate salts have become increasingly attractive due to recent advances and increased accessibility of ab initio calculations along with the prevalence of phosphorus in total synthesis efforts of medicinally relevant compounds. The ab initio efforts employed to provide insight on these compounds first included Gaussian density functional theory (DFT) calculations of charge densities and HOMO-LUMO gaps for the compounds. Significant charge disparities between the nitrogen-containing compounds and the analogous phosphorus-containing compounds were observed. In addition, the ab initio calculations of HOMO-LUMO gaps support prior studies which indicated that phosphonium tetrafluoroborate salts yielded higher reactivities than their ammonium analogs. The second part of the ab initio studies investigated predictive parameters for the regioselectivity of Diels-Alder addition to vinylammonium and vinylpyridinium tetrafluoroborate salts. Computational results show support in comparison to previous experimental results revealing that the trimethylammonium group was the stronger directing group compared to other substituent groups. Finally, phospha-Michael additions to triphenylphosphonium salts were investigated. Similar synthesis trends regarding E:Z ratios with various ester and amide substituents in these salts were observed. In contrast, there was significant deviations in E:Z product ratios and product type (seemingly a double-addition of the triphenylphosphonium salt) with the phospha-Michael addition to 3-butyn-2-one.